After having a hard time trying to hand solder a fpc connector and just giving up, I would suggest a more regular pin header sort of thing, maybe with two rows? (the ones with 2.54.mm pitch), or something through hole at least.
Otherwise, you are forcing people to either pay for assembly to jlc / pcbway and the sort or fork more money on tips, flux, braid, etc. Fpc is not handsolder friendly. That is, if you are aiming at the diy community.
While probably not a problem for a shop, it makes it more expensive for a few units and complicates (while probably only a bit) the ordering process.
Just my opinion after a bad soldering day.
edit: Note that I love the idea, would help to expand keyboards a great deal and can build the community.
When I started using home row mods I also experienced issues with shortcuts requiring ctrl (copy, paste, cut, dave, select all, close window, new tab, erc)
The solution I found was to map Ctrl in multiple positions only on the left hand so I do not need any contortions
This. I built the redox and you do need those resistors, but only on one side if I do not remember wrong
So you have the cirque glide point trackpads, pimoroni trackball, a nornal trackball and a thumbstick as pointes devices.
All work with qmk and I think there are branches on zmk with some of the above at different stages.
That is if you want to go ergo / custom. Look or ffkb, bastard keyboards and similar sellers for something less custom and pre built.
Depending on where you are, there is also the ploppy trackballs and mice which are open sourced... but thise are devices on their own.